1. Field of the Invention
The present invention relates to an air pump and more particular to an air pump having selectable low pressure and high pressure mode.
2. Description of the Related Art
In the prior art, U.S. Pat. No. 5,165,876 discloses a telescope air pump with two cylindrical bodies. One cylindrical body is fitted to one end of the other cylindrical body. There is a piston rod and a two-way air admission piston provided therein. So, the air pump can be divided into three air chambers. The air pump inflates an object when it is pushed or pulled.
U.S. Pat. No. 5,443,370 discloses a telescope air pump with three divisible cylinders. By increasing the route of push and pull, the air pump increases the air quantity in every stroke motion. The two adjacent cylinders can be locked together to make the big cylinder invalid and only two small cylinders keep working in high pressure mode.
U.S. Pat. No. 5,676,529 discloses a telescope air pump with two cylinders. There is an air passage provided between the big cylinder and the small cylinder. The air flows from the big cylinder to the small cylinder then goes into the head.
U.S. Pat. No. 6,027,319 discloses a telescope air pump with two cylinders. In this case, the big cylinder and the small cylinder inflate an object individually.
In order to operate in high pressure mode, patent 529 provides a knob as a pressure relief valve. The big cylinder at rear end will lose efficacy by turning the knob. Patent 319 provides a big cylinder at front and connects with the head at the front end. A pressure relief valve is provided at front and the big cylinder will lose efficacy by opening the valve also. But, there is a disadvantage of reorganization if the pressure relief valve operates by the way of rotation. In patent 529, when doing the inflation, the air in the big cylinder is pressed into the small cylinder first then goes inside the head. However, we will have P2>P1>P if the air pressure in the head is P, the air pressure in the small cylinder is P1 and the air pressure in the big cylinder is P2.
We will have P2=P1>P if we have the same assumption P, P1, P2 in patent 319. In according with the big cylinder and the small cylinder inflate an object individually; the air can inflate an object only when the air pressure is large then the air pressure in the head. If the air pressure in the big cylinder and the small cylinder is different, the low pressure part stops inflating an object for increasing air pressure and the high pressure part inflates an object first for releasing air pressure. The big cylinder and the small cylinder inflate an object at the same time when the air pressure is equal in both.
As the same reason as aforesaid, under the same operation power, patent 529 is easier to feel the resistance and needs to turn into the high pressure mode. Contrary to patent 319, in according to the two independent cylinders design, patent 319 provides a longer operation period in low pressure mode and the inflation efficiency is better then patent 529.
But the big cylinder in patent 319 is provided at front and connects with the head. This is more inconveniently then patent 529 to put the big cylinder into the very dense steel wires of the steel rim because that the small cylinder in patent 529 is provided at front.